I have a setup with Mac and Windows clients. The Mac users use Apple Mail and all users have an HTML signature with images. When a Mac user replies to an email the text from the initial message becomes an attachment with the name: untitled attachment001.html and the image from the signature becomes an attachment with the name: PastedGraphic.pdf.

Can this behaviour be avoided? How can I preserve the original email and signature?

I have managed the following. I discovered that it was due to the malformed signature. So I created a .webarchive file that contains the signature as I want to be. The signature appears correctly at both the file and the apple mail signature editor. But when I open to write a new mail only the borders of the signature image appear and there is an '?' inside the borders. What does this mean?

@peraueb8921 Re: your edits, is the question mark in a blue box? Do you see the question mark on the Mac before sending or in Windows after receipt?
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CajunLukeMar 16 '12 at 16:08

Almost...I made a step forward. I managed to see correlty the signature while I write a new mail. The message appears perfectly on Thunderbird but outlook says it cannot load 'cause a broken link:O
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py_scriptMar 17 '12 at 13:49

The outgoing mails are in rich text(does apple mail support outgoing in html?). I have tried to disable inline attachments but nothing happened. Seriously, Apple mail is so lame?
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py_scriptMar 15 '12 at 8:03

@peraueb8921 Is "use the same message format as the original message" checked?
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CajunLukeMar 15 '12 at 14:04

@peraueb8921 I believe that Mail's Rich Text is equivalent to HTML on Outlook.
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CajunLukeMar 15 '12 at 14:05

@peraueb8921 Have you tried Outlook for Mac? If you're having trouble with Mail, it might be worth it to switch to a different email client.
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CajunLukeMar 15 '12 at 14:07